This invention relates to an oxide superconductor lamination member and a method of manufacturing this member. More particularly, this invention relates to an oxide superconductor lamination member in which precious metal plates and an oxide superconductor layer are successively laminated on a base in such a manner that the precious metal plates overlap each other at their ends, and another oxide superconductor lamination member in which an oxide superconductor layer is laminated on each of precious metal bases and in which the precious metal bases overlap each other at their ends, and to a method of manufacturing these superconductor members.
Recently, oxide superconductors have attracted attention because of their high critical temperature and it is expected that they will be applied in various fields including that of electric power, that of magnetic resonance imaging apparatus and that of magnetic shield. To put oxide superconductors to practical use, instrumental elements or bases may be manufactured from oxide superconductors. On the other hand, a method of forming a layer of an oxide superconductor on an existing conventional base is known.
In a case where an oxide superconductor material is formed on a base, it is necessary to select a material for the base inactive in reaction with the oxide superconductor and having no effect of deteriorating the superconducting characteristics during firing of the oxide superconductor and/or to provide an intermediate inactive material. In addition, in a case where such a base material and/or intermediate inactive material are applied, a specific structure is required in which thermal stresses caused between the base or intermediate inactive material and the oxide superconductor are reduced and/or prevented from concentrating, since the present oxide superconductors are used by being cooled at liquid nitrogen temperature (77 K.).
Some superconductor members have been proposed. For example, Japanese Patent Laid-Open No. 63-305574 discloses an arrangement in which a stable material such as palladium (Pd), silver (Ag) or gold (Au) which does not react with other superconductor member materials is interposed between a base formed of, e.g., alumina, zirconia or a copper and an Y--Ba--Cu--O superconductor. Japanese Patent Laid-Open No. 1-173790 also discloses a superconductor member having a stabilized layer of, e.g., silver (Ag) formed on a Y--Ba--Cu--O superconductor.
In a case where an oxide superconductor is laminated on a precious metal plate such as a silver plate provided as a base or an intermediate layer to form a superconductor member large in size or length, it is necessary to connect several precious metal plates to each other at their ends to form one precious metal plate or precious metal cylinder constituting a base or intermediate layer having a large overall size, because it is difficult to form such as precious metal into the shape of one large plate. Also, in a case where a precious metal is elongated into a tape-like shape, the extent of elongation is limited for a reason in terms of manufacture so long as the metal is integrally formed, and it is necessary to connect end portions of precious metal tapes if a further elongation is required.
In particular, in the case of manufacture of a large cylindrical oxide superconductor member, a thick precious metal base is required and there is therefore the problem of a considerable increase in cost.
A type of oxide superconductor lamination member is therefore advantageous in terms of reduction in manufacture cost, which has a base formed of a material cheaper than precious metals and in which thin precious metal plates and an oxide superconductor layer are successively laminated on the base.
However, since the thickness of these precious metal plates is small and since it is difficult to manufacture one large precious metal plate, it is necessary to manufacture an intermediate layer precious metal plate having a large overall size by disposing several precious metal plates so that these metal plates overlap each other at their ends.
However, if a method of simply superposing end portions of precious metal plates is used or if a precious metal paste, glass, a superconductor material having the same composition as the superconductor layers and/or an inorganic adhesive consisting of a mixture of these materials is only applied to the overlapping or superposed portions, the following drawbacks are encountered. During firing for forming a superconductor layer on the base with a precious metal plate interposed therebetween, superconductor components penetrate through the overlap portions of the precious plates because the viscosity of the superconductor is very small at high temperatures, e.g., firing temperatures of 800.degree. to 1100.degree. C., and they react with an ordinary inorganic adhesive or a base material between the precious metal plates. Compounds produced by this reaction contaminate the superconductor layer so that the superconducting characteristics are considerably impaired. Moreover, the connected portions of the precious metal plates are easy to separate, and, as shown in FIG. 15(a), the thickness of the precious metal intermediate layer in the lap joint of precious metal plates 1, 1' is increased, so that a stepped portion X is formed at the lap-joint end of the precious metal plate on the oxide superconductor layer 3 side. The thickness of the superconductor layer 3 after firing is thereby made non-uniform at this stepped portion. In particular, if the superconductor member is manufactured by a firing process including the step of melting or partially melting the oxide superconductor, the non-uniformity of the thickness of the superconductor layer 3 after firing is considerably increased by the effect of the surface tension of the superconductor melt in the melting step. If the superconductor layer 3 has such a thickness non-uniformity at the stepped portion, the critical current thereof is reduced and stresses concentrate therein when the superconductor member is immersed in liquid nitrogen, resulting in occurrence of a crack 30 in the superconductor layer 3 at the stepped portion X, as shown in FIG. 15(b).